Fuel cell power systems convert a fuel and an oxidant to electricity. One type of fuel cell power system employs a proton exchange membrane (hereinafter “PEM”) to catalytically facilitate reaction of fuels (such as hydrogen) and oxidants (such as air or oxygen) to generate electricity. The PEM is a solid polymer electrolyte that facilitates transfer of protons from the anode to the cathode in each individual fuel cell of a stack of fuel cells normally deployed in a fuel cell power system.
In a typical fuel cell stack of a fuel cell power system, individual fuel cells provide channels through which various reactants and cooling fluids flow. Fuel cell plates may be unipolar, or a bipolar plate may be formed by combining a plurality of unipolar plates. Fuel cell plates may be designed with serpentine flow channels. Serpentine flow channels are desirable as they effectively distribute reactants over the active area of an operating fuel cell, thereby maximizing performance and stability. Movement of water from the flow channels to outlet manifolds of the fuel cell plates is caused by the flow of the reactants through the fuel cell. Drag forces cause the liquid water to flow through the channels until the liquid water exits the fuel cell through the outlet manifolds. However, when the fuel cell is operating at a lower power output, the velocity of the gas flow is too low to produce an effective drag force to transport the liquid water, and the liquid water accumulates in the flow channels.
A further limitation of relying on gas flow drag forces to remove the liquid water is that the drag forces may not be strong enough to effectively transport the liquid water creating pinning points that may cause the water to accumulate and pool, thereby stopping the water flow. Such pinning points are those commonly located where the channel outlets meet the fuel cell stack manifold.
Some current fuel cell assemblies utilize plates having hydrophilic surfaces. Water has been observed to form a film on the surface of the material and accumulate at the outlet of the flow channels and the perimeter of the plates. The water film can block the gas flow, which in turn reduces the driving force for removing liquid water and prevents the removal of the liquid water from the fuel cell stack. The accumulation of water can cause gas flow blockages or flow imbalances that can have negative impacts on the performance of the stack.
Further, the accumulated water may form ice in the fuel cell assembly. The presence of water and ice may affect the performance of the fuel cell assembly. During typical operation of the fuel cell assembly, waste heat from the fuel cell reaction heats the assembly and militates against vapor condensation and ice formation in the assembly. During a starting operation or low power operation of the fuel cell assembly in subzero temperatures, the condensed water in the flow channels of the fuel cell plates and at edges of the outlet manifolds may form ice within the fuel cell assembly. The ice formation may restrict reactant flow, resulting in a voltage loss.
It would be desirable to develop a fuel cell assembly with an improved means for removing liquid water from fuel cell gas flow channels of the fuel cell stack to minimize the accumulation of liquid water and ice in the fuel cell assembly.